Button Box/ Switch Allocation

Hi All,
I'm a recent convert from console racing where buttons/functions mapped to your wheel is the norm, to now trying to figure out which button should be mapped to which function whether on the wheel or button box and in which order on multiple sims.
I have a DSD Race King button box on its way and I realise that it's all personal preference but I would like your thoughts on which are the main functions I should have assigned to which type of button/placement on both this and the wheel (T500RS) before I get the Dymo machine out and start labelling everything up (memory is terrible). Currently I'm flitting between iRacing, Assetto Corsa, R3E and GSCE (haven't even fired up the PS3s/4 since discovering PC racing) so generic terms for same type of functions that can be used in all or most of these titles would be appreciated.

A bit of explanation might be needed though:
I use a program called Voice Attack (www.voiceattack.com) for some of the less critical commands, like jumping to a specific black box in iRacing, I'll just say relative to have the game change to the relative black box. I do however keep some hardware backup in case voice commands stops working mid race (hence the next/prev black box buttons on the wheel for instance), but it's been flawless so far.
On the CSW wheel I use the right joystick for traversing pit/in car menus, up/down on the stick for up/down or next/previous, push the stick for Select and rotate it for increase/decrease.
I use the left stick for Look Left/Right, but it's not labeled.

Finally, here's what each label means;

Wheel:
KERS/PTP = Boost button, Push To Pass
Pass L/R = Sends a chat message of pass left/right
Horn = not in use
PTT = Push To Talk, either ingame iRacing or TeamSpeak (I never use both at the same time)
VA = Voice Attack, I use a button to listen, you can have it listen non stop if you dont want a button.
LAUNCH = Launch control, boy have I ever used this... no I haven't
<- BOX -> = Prev / Next Black Box or display mode in other games

Kjell, thank you very much for the detailed info, definitely gives me a good starting point for assigning functions in regard to priority/ergonomics.
I didn't think I would need all the buttons initially but now with access to cars above the MX5 in iracing there are a whole raft of new tweaks and adjustments to do in car and pit strategies to worry about.
Unfortunately on night shift this week so haven't even managed to mount the box to the rig yet, but will put the time to good use "borrowing" the dymo machine at work to print out suitable labels based on your pics. Cheers